At about 3 weeks gestation, your cat's nipples will "pink up", ei, go a bright pink colour. If she has not had kittens before, you may find they are larger too. Some cats are also sick once or twice in early pregnancy, even they have morning sickness!! A cat's abdomen will start to enlarge at around 6 weeks of pregnancy.
Gestation period for a cat is average 65 days (9 weeks 2 days), but can be anywhere between 58 and 70 days.